1. Field of the Invention
This invention relates to a laser irradiation process comprising irradiating a pulse laser beam on a non-single crystal semiconductor film for annealing; in particular it relates to a laser irradiation process for forming a channel layer in a polycrystalline silicon thin film transistor which may be, for example, used for a liquid crystal display or a contact type image sensor.
2. Description of the Related Art
Recently, a thin film transistor comprising a polysilicon film as a channel layer on a glass substrate has been intensely developed for application in, for example, a liquid crystal display or a contact type image sensor. A laser annealing has been a common technique for preparing a polysilicon film in the light of reduction in a process temperature and improvement of a throughput; the process comprises forming a silicon film as a precursor on which an ultraviolet pulse laser beam is then irradiated to cause crystallization via fusion for forming a polycrystalline structure.
A common laser irradiation process is irradiation of a pulse laser beam having a rectangular or linear irradiation region.
JP-A 9-246183 has disclosed a process for irradiating a laser beam having a trapezoidal profile in a linewidth direction, which will be described with reference to FIG. 8. The process comprises irradiating a laser having a trapezoidal profile as shown in the figure only once to provide a polycrystalline structure. The energy density in the center of the trapezoidal beam profile is equal to or higher than a microcrystallization threshold for an amorphous semiconductor (amorphous silicon). There exists a region with a slightly lower energy than the microcrystallization threshold in the slope of the trapezoidal beam profile. It is believed that a polycrystalline semiconductor region with a large grain size can be formed.
This process may form a polycrystalline semiconductor region with a large grain size in a region with a slightly lower energy than the microcrystallization threshold, but the large crystal grains are randomly arranged and generally their size is widely distributed. Thus, when such a polycrystalline semiconductor region is used as, for example, a channel layer for a TFT (Thin Film Transistor), TFT properties such as a mobility may significantly vary.
The above publication has disclosed an irradiation process where a linear laser is scanned in its linewidth direction, i.e., a direction perpendicular to the line direction, which may provide a large area of polycrystalline silicon film. Thus, a variety of processes for irradiating a laser beam with a trapezoidal profile by scanning have been proposed. These processes, however, have a common problem that a crystal structure in a polycrystalline silicon film formed is poorly uniform. For example, Nouda et al, Shingaku Giho Vol. SDM92-112, p. 53 (1992) has disclosed that a beam end of a pulse laser beam may significantly vary the size of a crystal formed by the next irradiation, due to dependency of the state of the melted film by laser irradiation on the film structure before irradiation. In particular, a fused state is considerably changed in an interface between an already-irradiated region (crystallized region) and a non-irradiated region (amorphous region) when an amorphous silicon film is used as a precursor.
FIG. 9 shows a grain size distribution for a polycrystalline silicon structure formed by a laser annealing using a laser beam having a common trapezoidal energy density profile. FIG. 9(b) shows a grain size distribution in a polycrystalline region formed by irradiating a pulse laser having the profile shown in FIG. 9(a) on an amorphous silicon film. Then, a pulse laser beam is irradiated by scanning with a pitch x, so that the grain size distribution may vary as shown in FIG. 9(c). In the figure, a local minimum in a grain size can be observed in a region around the beam end in FIG. 9(b). Finally, a polycrystalline silicon film having a grain size distribution shown in FIG. 9(d) is formed, except the region where irradiation is initiated or stopped. In other words, there occurs unevenness in a grain size due to change in a crystal structure caused by the beam end. The grain size shown in FIG. 9(d) is an average size, and when such a trapezoidal beam profile is used, large crystal grains are randomly arranged, leading to a wide distribution of grain size as described above.